Summary of the Invention
The present invention relates to a scatterometer that can measure and quantify the scattering of light or other radiation from or through a test sample at a plurality of locations over a complete hemisphere.
Description of the Related Art
A scatterometer is a device that measures the intensity of light scattered by an object. For example, a theoretical perfect mirror would perfectly reflect a laser beam in a single direction. In reality the properties of the test sample (material properties as well as surface and body structure) will cause some portion of the light to be scattered in different directions known as scatter angles. As a result the scatter signal can vary by as many as fourteen decades over the hemisphere. It will normally peak in the direction associated with the reflected (or transmitted) specular beam, where it can change by several decades over just a few degrees. For many samples a serious technical issue is getting accurate measurements in both the specular and non-specular regions. Scatter can be measured in either the reflective hemisphere, or the transmissive hemisphere, or both. Scatterometers sample the reflected or transmitted light in steps that are close enough that software can estimate the light intensity between measurement locations. Typically more measurements are made close to the specular beam, where light levels are changing faster per unit angle, than at higher scatter angles. Most scatterometers in use today measure just a fraction of one hemisphere and are often limited to just the incident plane. Measurements are recorded as a function of angular position on the hemisphere in units that are defined by international standards. These scatter measurements are used in a variety of research, design, and manufacturing processes to directly quantify the optical properties of materials. In addition to measuring the optical performance of a material, scatter measurements are used to infer other physical information about the material composition and structure (such as surface roughness).
The prior art teaches a variety of systems that scan a single detector or employ multiple fixed detectors. These include systems that:                sample scatter in the incident plane and gather enough data to calculate in-plane scatter, for example, U.S. Pat. No. 5,241,369;        sample hemispherical scatter with a single detector and (slowly) sample enough data to calculate hemispherical scatter, for example, U.S. Pat. No. 7,349,096;        use screens and/or cameras to sample scatter over a limited portion of the hemisphere allowing scatter to be quickly calculated but only for a limited dynamic range, for example, U.S. Pat. No. 7,248,368; or        use complex systems of large fiber-optic bundles, or multiple fixed detectors, to coarsely sample hemispherical scattered light, for example, U.S. Pat. No. 5,313,542.        
In general these systems do not take enough data samples to calculate scatter over most of the hemisphere and/or are not able to cover the large dynamic range of scatter associated with optical surfaces. The fixed multiple detector systems have dynamic range problems when the incident angle is changed. The moving single detector systems have problems with measurement speed.